Saturday, June 01, 2013

Melzack and Katz, Pain. Part 6b: History of pain - what is in "Ref.4"?

The paper, Pain.

Part 1 First two sentences

Part 2 Pain is personal

Part 3a Pain is more than sensation: Backdrop

Part 3b Pain is not receptor stimulation

Part 3c: Pain depends on everything ever experienced by an individual

Part 4: Pain is a multidimensional experience across time
Part 5: Pain and purpose
Part 6a: Descartes and his era

Because this series is a long meander, I want to meander through that first paragraph of "A brief history of pain" a whole lot longer.
"The theory of pain we inherited in the 20th century was proposed by Descartes three centuries earlier. The impact of Descartes’ specificity theory was enormous. It influenced experiments on the anatomy and physiology of pain up to the first half of the 20th century (reviewed in Ref 4). This body of research is marked by a search for specific pain fibers and pathways and a pain center in the brain. The result was a concept of pain as a specific, direct-line sensory projection system."
My old, beloved, well-worn copy

I have the book, Challenge of Pain, which is Ref. 4, mentioned above. I've read it cover to cover. It's full of little post-it 
notes and underlining. 

Part Three, Theories of Pain, starts on page 147.

On the title page appears this quote by Donald Hebb, from 1973:

"The "real world" is a construct, and some of the peculiarities of scientific thought become more intelligible when this fact is recognized... Einstein himself in 1926 told Heisenberg it was nonsense to found a theory on observable facts alone: "In reality the very opposite happens. It is theory which decides what we can observe." "
Melzack and Wall say,
"A theory is primarily an attempted solution to a puzzle or problem - like a guess made by a detective presented with an array of clues in a mystery."

Several theories about pain are then examined. They begin with specificity theory, lay it out for all to ponder:

Specificity theory

  • traditional
  • often taught as fact rather than theory
  • presented as if major answers to pain problems had already been discovered, and all that remained were minor therapy questions
  • very powerful theory nonetheless - has given rise to a lot of excellent research and some effective forms of treatment
  • has several basic flaws however:
  • proposes a specific pain system that carries messages from skin receptors to a pain center in the brain (Descartes)
  • contains major assumptions

How it evolved in the last three centuries

1. Müller's doctrine of specific nerve energies (1842): 

  • he made formal statement that the brain receives information about external objects only by way of the sensory nerves.
  • he implied that activity in nerves represented coded or symbolic data concerning the stimulus object
  • he recognized only the 5 classic senses: seeing, hearing, taste , smell, touch
  • line of research was for a terminal area in the brain for each sense
  • visual and auditory centers were found first: an assumption was made that these were the seat of seeing, hearing: physiologists of the day were convinced this was truth
  • Max von Frey proposed a theory of cutaneous senses

2. Von Frey's theory (1894)

Von Frey put together three kinds of information: 
  • Müller's doctrine, which he expanded to include 4 kinds of cutaneous modality - touch, warmth, cold, and pain, each with it's own dedicated line to the brain center responsible for the appropriate sensation
  • spot-like distribution of heat or cold sensitivity on skin - from that he assumed skin would comprise a mosaic of touch, cold, warmth, and pain
  • previous discovery of skin receptors (he was mostly interested in free nerve endings but had opinions on the rest)
He created an interesting example of scientific deduction:
  • since free nerve endings are the most common sort, and
  • pain spots are found everywhere,
  • therefore free nerve endings are pain fibres..
  • furthermore, Meissner corpuscles were touch receptors, Krause end-bulbs were cold receptors, and Ruffinis were warmth receptors
3. Others furthered his work
Many experiments, reviewed by Bishop 1946, Rose and Mountcastle 1959, Sinclair 1982, provided evidence for the idea that some one-to-one relationship exists between receptor type, fibre size, and quality of experience.
  • very literal interpretation of Müller's doctrine of specific nerve energies
  • specificity theorists talk about A-delta-pain, C-fibre-pain, touch fibres and cold fibres as though each had a straight through transmission path to a specific brain centre
  • spinothalamic tract was deemed to be the "pain" pathway
  • thalamus is deemed to contain a "pain" centrer: it is assumed the cortex can exert inhibitory control

Melzack and Wall point out Von Frey's three main theoretical assumptions

1. Receptor specialization
  • the idea "that receptors are specialized, is physiological in nature and has achieved the proportions of a genuine biological law." (p. 154)
  • Sherrington (1900, 1906) refined this in terms of "adequate stimulus"- this interpretation is acceptable to those interested in the eventual psychological experience. 
2. Anatomical assumption
  • the most obviously incorrect and least relevant aspect of Von Frey's theory
  • no one can find any support for this assumption 
"Von Frey assumed that a single morphologically specific receptor lay beneath each sensory spot on the skin and he assigned a definite receptor type to each of the four modalities. The crucial experiment of making a histological examination of the skin under carefully mapped temperature spots has been performed at least a dozen times (see Melzack and Wall 1962) without a single investigator finding any support for Von Frey's anatomical correlations." (p. 155) 
3. Psychological assumption
  • the idea that there is any one-to-one correspondence between a single stimulus dimension or given type of skin receptor, and a psychological dimension of somaesthetic experience, is the most questionable Von Frey assumption
  • is essentially similar to Descartes concept of pain from 1600's
  • depicts a fixed, straight-through conceptual nervous system
"It is precisely this facet of the specificity concept, which imputes a direct, invariant relationship between a psychological sensory dimension and a physical stimulus dimension, that has led to attempts at repudiation of the doctrine of specificity in its entirety." p 155

Next post: the obvious flaws in specificity theory, from the vantage point of psychology, holes so big in it you could drive a truck through them. 

Well, maybe not quite this big,  but big enough, certainly, to cause concern

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